Li-Ru/Al2O3 as Dual Function Materials for Combined CO2 Capture and Methanation

The alarming increasing CO2 levels in the atmosphere with the consequent effect on climate change pushes research into the development of methods for both decreasing CO2 emissions and for the capture of CO2 released. Conversion of captured CO2 into added-value chemicals and fuels can represent a smart solution. Nevertheless, the ambitious goals of net-zero greenhouse gas emissions in the European Union by 2050 can be reached only through the development of effective Carbon Capture and Utilization (CCU) technologies. One of these processes is based on the reaction of CO2 with hydrogen from renewable sources producing methane that can be used as synthetic substitute of natural gas (SNG). This Power-to-Gas process also tackles the issue related to the fluctuation and intermittence of wind and solar energy converting surplus power into SNG which can be injected into the existing gas distribution grid. The combined capture and hydrogenation of CO2 to methane represents an innovative solution in the context of the Power to Gas technology. It consists of a catalyst-assisted chemical looping process including a step of CO2 capture from a point-source over a Dual Function Material (DFM) composed of an adsorbent which also serves as support for an active metal catalyst, producing CH4 upon reaction with H2 from renewable sources regenerating the material at the same time. The DFM must fulfil a number of stringent characteristics, including high catalytic activity and selectivity at low temperature, high mechanical stability and long durability under cyclic operation, easy reducibility, large and selective CO2 adsorption capability and easy desorption/regeneration. As a consequence, the synergism between the storage and the activation on the catalytic element is the key parameter to the development of more effective DFMs. In this work, mechanistic aspects involved in cycled CO2 capture and methanation over highly performing Lithium-Ruthenium/Al2O3 DFMs are investigated. DFM with low fixed (ca. 1% wt.) Ru loading and variable Li contents (up to 5% wt.) were prepared by impregnation of commercial ?-Al2O3 spheres (1mm) of high mechanical stability. They were characterized by BET, PSD, XRD, H2 chemisorption, CO2-TPD, TG-MS, H2-TPSRx, in situ DRIFT and CO2 catalytic methanation under continuous flow conditions. Transient CO2 storage/methanation cycles were studied in a fixed bed reactor that was operated in the temperature range 250 - 350 °C with alternate feed conditions of variable duration to identify those maximizing methane productivity and selectivity. Lithium, dispersed on Ru/Al2O3 spheres starting form nitrate solution, reacted with alumina leading to the formation of mixed Li-aluminate phases (i.e. LiAl5O8, ?-LiAlO2 and Li2Al4(CO3)(OH)12-3H2O) that can capture and store significant amounts of CO2 at moderate temperatures (up to 400 °C). The formation of mixed aluminate phases, peculiar of the Li-Al2O3 system, prevents the formation of highly stable carbonate species, often reported with other alkali or alkaline-promoted DFMs, which are difficult to hydrogenate, particularly at low Ru loadings. Indeed, in situ DRIFT and TG-MS results revealed that CO2 was chemisorbed mostly as bidentate carbonates, while releasing water in the gas phase, whilst further amounts of CO2 can be stored, at a slower rate, as weakly bonded bicarbonates. In-situ DRIFTs confirmed the existence of an effective spill-over mechanism of CO2 from Li-Al oxide to adjacent Ru nanoparticles where it can easily react with co-adsorbed H to form methane while regenerating the original adsorption site. This occurs in a temperature range far lower than that required for the decomposition of the highly stable alkaline carbonates, thus creating a favourable synergism between